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Ignition timing, in a spark ignition internal combustion engine, is the process of setting the time that a spark will occur in the combustion chamber (during the power stroke) relative to piston position and crankshaft angular velocity.
Setting the correct ignition timing is crucial in the performance of an engine. The ignition timing affects many variables including engine longevity, fuel economy, and engine power. Modern engines that are controlled by an engine control unit use a computer to control the timing throughout the engine's RPM range. Older engines that use mechanical spark distributors rely on inertia (by using rotating weights and springs) and manifold vacuum in order to set the ignition timing throughout the engine's RPM range. There are many factors that influence ignition timing. These include which type of ignition system is used, engine speed and load, which components are used in the ignition system, and the settings of the ignition system components. Usually, any major engine changes or upgrades will require a change to the ignition timing settings of the engine.

History
The ignition systems of older, non-computer controlled engines consists of a mechanical spark distributor which distributes spark to cylinders based on static (initial) timing, mechanical timing advance, and vacuum timing advance. In 1972, Chrysler introduced the electronic ignition system that replaced the mechanical contact breaker ("points"). This provided for a stronger ignition spark, as well as virtually eliminating variations in ignition timing due to wear associated with breaker points.
Actual electronic control of ignition timing was introduced a few years later in 1979 with the Bosch Motronic engine management system. This system introduced simultaneous control of both the ignition timing and fuel delivery, and is the basis for modern systems.

Setting The Ignition Timing
"Timing advance" refers to the number of degrees before top dead centre (BTDC) that the spark will ignite the air-fuel mixture in the combustion chamber during the compression stroke. Retarded timing can be defined as; changing the timing so that fuel ignition happens later than the manufacturer's specified time. If the ignition timing, specified by the manufacturer, was to be set at 12 degrees BTDC and it was adjusted to a number lower than 12 degrees BTDC, it would be retarded. In a classic ignition system with breaker points, the basic timing can be set statically using a test light or dynamically using a timing light.
Timing advance is required because it takes time to burn the air-fuel mixture. Igniting the mixture before the piston reaches top dead centre (TDC) will allow the mixture to fully burn soon after the piston reaches TDC. If the air-fuel mixture is ignited at the correct time, maximum pressure in the cylinder will occur sometime after the piston reaches TDC allowing the ignited mixture to push the piston down the cylinder with the greatest force. Ideally, the time at which the mixture should be fully burnt is about 20 degrees ATDC. This will utilize the engine's power producing potential. If the ignition spark occurs at a position that is too advanced relative to piston position, the rapidly expanding air-fuel mixture can actually push against the piston still moving up, causing detonation and lost power. If the spark occurs too retarded relative to the piston position, maximum cylinder pressure will occur after the piston is already travelling too far down the cylinder. This results in lost power, high emissions, and unburned fuel.
The ignition timing will need to become increasingly advanced (relative to TDC) as the engine speed increases so that the air-fuel mixture has the correct amount of time to fully burn. As the engine speed increases, the time available to burn the mixture decreases but the burning itself proceeds at the same speed, it needs to be started increasingly earlier to complete in time. Poor volumetric efficiency at lower engine speeds also requires increased advancement of ignition timing. The correct timing advance for a given engine speed will allow for maximum cylinder pressure to be achieved at the correct crankshaft angular position. When setting the timing for an automobile engine, the factory timing setting can usually be found on a sticker in the engine bay.
The ignition timing is also dependent on the load of the engine with more load (larger throttle opening) requiring less advance (the mixture burns faster). Also it is dependent on the temperature of the engine with lower temperature allowing for more advance. The speed with which the mixture burns depends also on the Octane rating of the fuel and also the air/fuel ratio.

Dyno Tuning
Setting the ignition timing while monitoring engine power output with a dynamometer is one way to correctly set the ignition timing. After advancing or retarding the timing, a corresponding change in power output will usually occur. A load type dynamometer is the best way to accomplish this as the engine can be held at a steady speed and load while the timing is adjusted for maximum output.
Using a knock sensor to find the correct timing is one method used to tune an engine. In this method, the timing is advanced until knock occurs. The timing is then retarded one or two degrees and set there. After achieving the desired power characteristics for a given engine load/rpm, the spark plugs should be inspected for signs of engine detonation. If there are any such signs, the ignition timing should be retarded until there are none.

Mechanical Ignition Systems
Mechanical ignition systems use a mechanical spark distributor to distribute a high voltage current to the correct spark plug at the correct time. In order to set an initial timing advance or timing retard for an engine, the engine is allowed to idle and the distributor is adjusted to achieve the best ignition timing for the engine at idle speed. This process is called 'setting the base advance'. There are two methods of increasing timing advance past the base advance. The advances achieved by these methods are added to the base advance number in order to achieve a total timing advance number.

Mechanical Timing Advance
An increasing mechanical advancement of the timing takes place with increasing engine speed. This is possible by using the law of inertia. Weights and springs inside the distributor rotate and affect the timing advance according to engine speed by altering the angular position of the timing sensor shaft with respect to the actual engine position. This type of timing advance is also referred to as centrifugal timing advance. The amount of mechanical advance is dependent solely on the speed at which the distributor is rotating. In a 2-stroke engine, this is the same as engine RPM. In a 4-stroke engine, this is half the engine RPM. The relationship between advance in degrees and distributor RPM can be drawn as a simple 2-dimensional graph.
Lighter weights or heavier springs can be used to reduce the timing advance at lower engine rpm's. Heavier weights or lighter springs can be used to advance the timing at lower engine rpm's. Usually, at some point in the engine's rpm range, these weights contact their travel limits, and the amount of centrifugal ignition advance is then fixed above that rpm.

Vacuum Timing Advance
The second method used to advance the ignition timing is called vacuum timing advance. This method is almost always used in addition to mechanical timing advance. It generally increases fuel economy and driveability, particularly at lean mixtures. Vacuum advance works by using a manifold vacuum source to advance the timing at low to mid engine load conditions by rotating the position sensor (contact points, hall effect or optical sensor, reluctor stator, etc) mounting plate in the distributor with respect to the distributor shaft. Vacuum advance is diminished at wide open throttle (WOT), causing the timing advance to return to the base advance in addition to the mechanical advance.
One source for vacuum advance is a small opening located in the wall of the throttle body or carburettor adjacent to but slightly upstream of the edge of the throttle plate. This is called a ported vacuum. The effect of having the opening here is that there is little or no vacuum at idle. Other vehicles use vacuum directly from the intake manifold. This provides full engine vacuum (and hence, full vacuum advance) at idle.
On some vehicles, a temperature sensing switch will apply manifold vacuum to the vacuum advance system when the engine is hot or cold, and ported vacuum at normal operating temperature. This is a version of emissions control, the ported vacuum allowed carburettor adjustments for a leaner idle mixture. At high engine temperature, the increased advance raised engine speed to allow the cooling system to operate more efficiently. At low temperature the advance allowed the enriched warm-up mixture to burn more completely, providing better cold-engine running.
Electrical or mechanical switches may be used to prevent or alter vacuum advance under certain conditions. Early emissions electronics would engage some in relation to oxygen sensor signals or activation of emissions related equipment. It was also common to prevent some or all of the vacuum advance in certain gears to prevent detonation due to lean-burning engines.
Most vehicle manufacturers specify that the vacuum line for the vacuum advance (if equipped) should be disconnected and plugged when adjusting the initial advance setting, which is usually specified in a workshop manual. Be careful when turning the distributor while the engine is running because deteriorated spark plug wires can deliver a painful shock.

Electronic Ignition Systems
Newer engines typically use electronic ignition systems (ignition controlled by a computer). The computer has a timing map which is a table with engine speed on one axis and engine load on another axis. Timing advance values are inserted in this table. The computer will send a signal to the ignition coil at the indicated time in the timing map in order to spark the spark plug. Most computers from original equipment manufacturers (OEM) are not able to be modified so changing the timing advance curve is not possible. Overall timing changes are still possible, depending on the engine design. Aftermarket engine control units allow the tuner to make changes to the timing map. This allows the timing to be advanced or retarded based on various engine applications.